More particularly, the invention relates to such assembly, or tactile pressure sensor, used in a push button for detecting activation of the button.
Generally, a pressure sensitive transducer assembly comprises a force sensing resistor with connection means, said force sensing resistor comprising: first and second substrates each having inner surfaces, wherein at least one of the substrates is flexible in order to move towards the other one of the substrates in response to an applied force; at least a first and a second electrically conductive traces on the inner surface of the first substrate, said first trace including a first set of fingers interdigitated with a second set of fingers pertaining to the second trace so as to define together a sensitive area on the inner surface of the first substrate, the first set of fingers and the second set of fingers being electrically separated from one another on the inner face of the first substrate; and a resistive layer on the inner surface of the second substrate such that the resistive layer is facing the sensitive area.
In the absence of applied force on the substrates, a first electrical signal indicative of a rest state is produced by a control circuit applying a reference voltage between first and second traces.
In response to a force applied to a flexible one of the substrates, the resistive layer contacts and electrically connects fingers of the first set and fingers of the second set together with a resistance dependent upon resistivity of the resistive layer and dependent upon the applied force. Thus, the control circuit is able to produce a second electrical signal indicative of the applied force when applying said reference voltage between first and second traces.
Such a pressure sensitive transducer assembly is already described in US2006/0007172A1 wherein it comprises a force sensing resistor working without preload in its rest state, an air gap preventing contact between both substrates. Such a force sensing resistor will be designated hereafter as standard force sensing resistor.
WO2009/070503A1 discloses an alternative embodiment wherein the force sensing resistor is designed to work under preload condition in its rest state, the substrates being touching each other even in the absence of an externally applied force. Such a force sensing resistor will be designated hereafter as preloaded force sensing resistor.
Current transducer assemblies are not fully satisfactory because it is not possible to detect easily when the assembly is disconnected from the control circuit as it provides the same output voltage for different situations.
For assemblies using standard force sensing resistor, the output voltage provided to the control circuit through its connection means is substantially equal to a constant voltage both when it is disconnected and in its rest state (without external applied force).
For assemblies using preloaded force sensing resistor, the output voltage provided to the control circuit through its connection means is substantially equal to a constant voltage both when it is disconnected and when the preload condition is lost.
It is particularly important to be able to detect disconnection of the assembly in applications such as in the automotive industry, as a high number of components are assembled simultaneously and the function controlled by the button through the pressure sensitive transducer assembly can be critical.
It is another important issue to be able to make a diagnostic about the status of the force sensing resistor in order to detect abnormalities such as bad contact between the force sensing resistor and the central unit, loss of preload condition, electronics failure, disconnection of the force sensing resistor connector or operation out of the normal operating range of the force sensing resistor.
Common solutions provide either additional contact pins or wires for test purpose by the central unit for checking continuity between the pins and connection of the force sensing resistor and its characteristic, a fixed resistive layer directly printed on the force sensing resistor with an analog driver or camera testing at the end of line tester (EOLT) or in-line tester to check correct insertion of the connector portion of the force sensing resistor into the connection means by using the shape detection principle.
However, adding new pins for test purpose implies a higher cost for the connector and for the force sensing resistor. Visual check and camera testing either EOLT or in-line also increase costs and assembly time and cannot easily implemented due to a small space and bad visibility of the connector area within the product. Further, the only electronics failure that can be detected is a short to ground or to power supply. Any other electronics failure in the force sensing resistor driver will not be detected. Moreover, these common solutions do not permit to distinguish an electronics failure from a use of the force sensing resistor out of its normal operating range. Resolution of an analog driver is not sufficient to detect loss of preload condition. Although contact between the connector portion of the force sensing resistor and the connector may be checked, correct insertion still needs to be checked.